DTIC ADA548686: Probing and Manipulation of Coherent States in Ferromagnetic Narrow Gap Semiconductors with an Eye Towards Developing Concepts for New Device Functionalities

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AFRL-OSR-VA-TR-2011-0025

Probing and Manipulation of Coherent States in Ferromagnetic
Narrow Gap Semiconductors with an Eye Towards Developing
Concepts for New Device Funcionalities

Giti Khodaparast

Virginia Poly Technique and State University (Va. Tech)

JANUARY 2011
Final Report

DISTRIBUTION A: Distribution approved for public release.

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Probing And Manipulation Of Coherent States In
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Ferromagnetic Narrow Gap Semiconductors With An Eye
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Final Report: Cover Page

Principle Investigator: Prof. Giti A. Khodaparast

Institution: Virginia Poly Technique and State University (Virginia Tech.)
209A, Robeson Hall, Blacksburg, VA, 24061
Tel: [phone]
Fax: [phone]
http://www.phys.vt.edu/~khodapar/Giti-group.htm
Email:[email protected]

Agreement Number: Grant number FA9550-07-1-0086

Title: Probing and Manipulation of Coherent States in Ferromagnetic Narrow Gap
Semiconductors with an Eye Towards Developing Concepts for New Device
Functionalities

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Status of efforts:

The goal of this project has been to study coherent phenomena in narrow gap semiconductors
(NGSs) including III-V ferromagnetic structures, such as InMnAs and InMnSb, with an emphasis on
dynamical aspects, such as charge and spin dynamics. In light of the growing interest in spin-related
phenomena and devices, there is now renewed interest in the science and engineering of NGSs and our
results are important for understanding the electronic and magnetic states in these material systems.
Narrow gap semiconductors have significant potentials for applications in infrared spin photonics and in
spin transport devices due to small energy gap, and much higher carrier mobility than in other III–Mn–V
semiconductors.
I have developed strong collaborations with several materials scientists and user facilities. Our recent
observations have been reported in new publications and several talks and poster presentations. Our
activities will advance the following AFOSR missions:

a. Fundamental understanding of materials that change their physical characteristics in
response to an external stimulus;
b. Multifunctional materials;
c. Fundamental research of electronic, photonic, and magnetic properties of solid state
materials, structures, and devices;
d. Developing concepts for advanced devices.

Accomplishments/ New Findings:

After the last year report my group pursued several new experiments and the examples are
presented in this final report. The publications on the basis of the recent observations are under
preparation. Several time resolved differential transmission (TRDT) schemes were employed to provide
insight into the time scales and the nature of microscopic interactions in metalorganic vapor phase epitaxy
(MOVPE) grown InMnAs and InMnSb. The MOVPE grown InMnAs structure, was an 800 nm thick film
with a Mn content of ~ 4 %, a hole concentration p=1.35x1018 cm-3, and a mobility of 142 cm2/Vs. The
200 nm thick InMnSb film has a hole concentration p=3x1019 cm-3, and a mobility of ~198 cm2/Vs with
the Mn content of 3.7 %.

Most of the understanding of the carrier relaxation in the MBE grown narrow gap (III,Mn)V
ferromagnetic structures has been on the basis of two color differential reflectivity spectroscopy with
pump and probe pulses ranging from 1.2-2μm and 650-850 nm, respectively [1].The information extracted
from the differential reflectivity can be affected by multi-reflections in the multi-layer structures. In this
work, our TRDT measurements demonstrate complex and tunable dynamics of the photo-excited carriers
in these ferromagnetic films which have not been reported before.

Figure 1 demonstrates the examples of two-color TRDT, where the pump and probe pulses were
tuned at 800 nm and 3.467 m, respectively. The TRDT in these two samples demonstrate different
relaxation patterns. The differential transmission change, in InMnSb ia about 15%; whereas, for InMnAs
under the same experimental conditions, the change exceeds 50%. In addition, the fast component of the
carrier relaxation in the InMnSb film is rather different compared to the InMnAs and the photo-induce
carriers fully relax in the time scale of 12 ps. In both samples 800 nm pump pulses can generate electrons
that are energetic to have the possibility scattering between the X, L, and Γ valleys in the conduction band
[2-3]. Our earlier degenerate MIR pump/probe measurements demonstrated different relaxation dynamic
possibly influenced by non-linear effects involved in a degenerate scheme [4-5].

2
50
InMnSb InMnAs
Pump = 800 nm Pump = 800nm,
30 Probe = 3.467µm 40 Probe = 3.467 µm
% %
0 0 30
T20 T
T)/ T)/
0 - 0 - 20
T10 T
( (
10
0
a) 0 b)
0 4 8 12 0 5 10 15 20 25
Time Delay (ps) Time Delay (ps)

Fig. 1: Two-color differential transmission measurements in ferromagnetic films, at 290 K. The pump/probe pulses were
800 nm and 3.467 μm, respectively. In case of InMnSb, the photo-induced carriers were relaxed in the time scale of 12 ps;
whereas, in the InMnAs the carriers were not fully relaxed in a time scale longer than 25 ps.

As shown in Fig. 2, when the order of the pump and probe pulses were reversed and tuned to
3.467 μm and 800 nm, respectively, the differential reflectivity as a function of time delay in the InMnAs
was measured. The initial sharp decrease in the differential reflectivity can result from free carrier Drude
absorption whereas; the alteration of the dielectric function of the film through changes in the electron
and hole distribution functions, can be responsible for the observed sign change of the differential
reflectivity. A similar fast carrier dynamics and the sign change in the transient reflectivity have been
reported in MBE grown InMnAs and InAs films [1,6]. The possibility of increasing the relaxation time of
the photo-excited carriers by injection carrier above the gap into the satellite valley can be important in
developing devices on the basis of these ferromagnetic materials.
0.4
0.2
%
0.0

0
R
-0.2
/
0) InMnAs
R-0.4 Pump 3.467µm
-
R-0.6 Probe 800 nm
(
290 K
-0.8
-1.0
-2 0 2 4 6 8 10
Time Delay (ps)

Fig. 2: Two-color normalized transient reflectivity in ferromagnetic films, at 290 K. The pump/probe orders have been
reversed compared to Fig. 2. A similar fast carrier dynamics and the sign change in the transient reflectivity have been
reported in MBE grown InMnAs. The alteration of the dielectric function of the film through changes in the electron and
hole distribution functions, can be responsible for the sign change.
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In our measurements, the MIR laser source was a difference frequency generator (DFG), which mixes
the signal and idler beams from an optical parametric amplifier (OPA). The OPA itself was pumped by an
amplified Ti:sapphire oscillator with a repetition rate of 1 KHz. The pulses had a duration of ~ 100 fs
defining the resolution of the measurements. The differential transmissivity as a function of the time delay
between the pump and probe pulses, using a liquid N -cooled MCT detector was measured. Similarly,
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differential transmission measurements can extract the carrier relaxation time.

In addition to work at Virginia Tech, my group has been collaborating with large-scale facilities,
at two national labs (The National High Magnetic Field Labs in Florida and Institute for Solid State
Physics in Japan), to provide information on the properties of ferromagnetic material systems. An
example of these collaborative efforts is presented here.

Cyclotron Resonance at High Magnetic Fields

The carrier-mediated ferromagnetism in magnetic III-V semiconductors has provided a novel
system to study the physics of itinerant carriers interacting with localized spins of the magnetic impurities.
The nature of the carriers mediating ferromagnetism has been an open question. This fact motivated us to
probe Cyclotron Resonance (CR) in the ferromagnetic structures to understand the nature of the carrier
states. CR is a direct and accurate method for determining the effective mass and therefore the energy
dispersion of carriers. In DC transport measurements it is difficult to extract the contributions of the mass
and scattering time independently. Since these magnetic semiconductors usually have low carrier
mobilities, a very high magnetic field is necessary to observe cyclotron resonance (CR) (i.e., ωτ > 1
c

Ferromagnetic semiconductors are important materials for development of spintronic devices.
While effort in this area was made primarily on GaMnAs, other ferromagnetic III-Mn-V alloys have also
been developed, including the narrow gap ferromagnetic alloys InMnAs and InMnSb. For example,
important advances have now been made in the MOVPE growth of these latter systems [7-8]. Investigation
of the electronic structure of III-Mn-V alloys by techniques such as the cyclotron resonance (CR) can
shed important light on the origin of ferromagnetism
and the p-d exchange interaction in III-Mn-V systems.
In this work we report on CR experiments carried out
on the ferromagnetic InMnAs and InMnSb films, on
which clear resonance signals have been successfully
observed in high magnetic fields.

In Mn As films were grown by MOVPE at the
1-x x
Northwestern University and In Mn Sb films at the
1-x x
University of Notre Dame by low temperature MBE [9].
The Mn content (x), thickness (d), and hole
concentration (p) are x~0.02, d=200 nm, p~2×1018 cm-3
for In Mn As, and x~0.02, d=230 nm, p~1×1020 cm-3
1-x x
for In Mn Sb. The T ’s are about 320 K and 10 K in
1-x x c
In Mn As and In Mn Sb, respectively. CR
1-x x 1-x x
measurements were performed using CO and H O
2 2
lasers, providing laser radiation at 10.6, 10.7, and
16.9 μm wavelengths. Magnetic fields exceeding 100 T Fig. 3: Cyclotron resonance spectra of
were generated by a single turn coil technique. InMnAs and InMnsb.

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Figure 3 shows the CR spectra at 121 K. The effective masses deduced from these resonance peak
positions are 0.037m for InMnAs and 0.051m for InMnSb. The observed effective mass in InMnAs is
0 0
consistent with the heavy hole (HH) mass reported in an earlier CR study of p-type InMnAs with T of ~
c
10 K [10]. This HH effective mass is quite different from the classical HH band edge mass in InAs
(0.35m ) because of the known quantum effect at high magnetic fields with for small Landau level
0
indices (n = 0,1,2,..) [10]. The CR in ferromagnetic InMnSb was observed for the first time. The
deduced mass is also considerably smaller than the band edge HH mass of InSb (0.32m ), similar to the
0
result obtained for InMnAs. Detailed theoretical calculations will provide information on the carrier
states in the valence band of these ferromagnetic alloy systems.

Personnel Supported

Graduate Students supported: Matt Frazier (Currently a Post Doctoral associate at the Physics Department,
University of Maryland), Mithun Bhowmick, Travis Merritt. In addition, three undergraduate students
(Thomas How, Jonathan Cates and Jose Umanzor-Alvarez) were partially supported.

Publications Resulted from AFOSR Support

1- M. Frazier, J. G. Cates, J. A. Waugh, J. J. Heremans, M. B. Santos, X. Liu, and G. A.
Khodaparst,” Photoinduced spin-polarized current in InSb –based structures, J. App. Phys. 106,
103513 (2009).

2- Invited Paper: Giti A. Khodaparast, Matthew Frazier, Rajeev N. Kini, Kanowkan Nontapot,
Tetsuya D. Mishima, Michael Santos, Bruce W. Wessels Probe of Coherent and Quantum States
in Narrow-Gap Based Semiconductors with Strong Spin-Orbit Coupling Proc. of SPIE 7760,
77600x-1 (2010)

3- M. Bhowmick, R. N. Kini, K. Nontapot, N. Goel, S. J. Chung, T. D. Mishima, M. B. Santos, G.
A. Khodaparast, “Probe of Interband Relaxations of Photo-excited and Carriers and Spins in
InSb Based Quantum Wells”, Physics Procedia (3) 1161-1165 (2010).

4- Invited Paper: Giti A. Khodaparast, Matthew Frazier, Rajeev N. Kini, Kanowkan Nontapot,
Tetsuya D. Mishima, Michael Santos, Bruce W. Wessels, “Probe of Coherent and Quantum
States in Narrow-Gap Semiconductors in the presence of strong Spin-Orbit Coupling”. Proc.
SPIE 7608, 76080O-1 (2010)

5- M. Bhowmick, T. R. Merritt, K. Nontapot, B. W. Wessels, O. Drachenko, G. A. Khodaparast,
“Time Resolved Spectroscopy of InMnAs Using Differential Transmission Technique in Mid-
Infrared”, Physics Procedia, (3) 1167-1170 (2010).

6- R. N. Kini , K. Nontapot, G. A. Khodaparast, L. J. Guido, R.E. Welser “Time Resolved
Measurements of Spin and Carrier Dynamics in InAs Films” , J. of Applied Physics 103, 064318
(2008). Selected for April 2008 issue of Virtual Journal of Ultrafast Science.

7- M. Frazier, Nontapot K., Kini R., Khodaparast G. A., Wojtowicz T., Liu X., Furdyna J. K,”
Time Resolved Magneto-Optical Studies of Ferromagnetic InMnSb Films”, Appl. Phys. Lett., 92,
061911 (2008).

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8- Nontapot K., Kini R. N., Gifford A., Merritt T. R., Khodaparast G. A., Wojtowicz T., Liu X.,
Furdyna J. K., “Relaxation of Photoinduced Spins and Carriers in Ferromagnetic InMnSb Films,
Appl. Phys. Lett. 90, 143109, 2007
Selected for the May 2007 issue of Virtual Journal of Ultrafast Science and for the April 16, 2007
issue of Virtual Journal of Nanoscale Science & Technology.

Interactions/Transitions:

Several presentation introduce the research activities pursued in this work including: Two talks at the
International Conference on Narrow Gap Semiconductors (July 2005 and July 2007), one talk at the 2008
Electronic Materials Conference, and posters at the Aspen 2007 Winter Conference on Condensed Matter
and 2008 Magnetic Excitation in Semiconductors Symposium. Members of Dr. Khodaparast’s team gave
contributed talks at the APS March meeting [one in 2006, four in 2007, two in 2008, three in 2009, and
two in 2010] and presented two posters at the 14th International Conference of Narrow Gap
Semiconductors in Japan, 2010. One contributed talk was given at the 2010 Electronic Materials , and a
poster was presented at The 19th International Conference on the Application of High Magnetic Fields in
Semiconductor Physics and Nanotechnology by Prof. Y. Matsuda (Institute of Solid State Physics, Japan),
who is a regular collaborator. In addition to several colloquiums, Dr. Khodaparast’s presented two recent
invited talks in photonic West (Jan. 2010) and SPIE –SPIN-III (August 2010) conferences.

References:
1- J. Wang, C. Sun , Y. Hashimoto , J. Kono , G. A. Khodaparast , L. Cywinski, L. J. Sham., G. D.
Sanders., C. J. Stanton and H. Munekata, Journal of Physics: Condens. Matter, 18 (2006) R501; J.
Wang, G. A. Khodaparast, J. Kono, A. Oiwa, and H. Munekata, Journal of Modern Optics, 51 (2004)
2771.
2- Ulman, M., Bailey, D. W., Acioli, L. H., Vallee, F. G., Stanton, C. J., Ippen, E. P., and Fujimoto, J.
G., Phys. Rev. B 47, 10267-10278 (1993).

3- Kuznetsov, A. V., Kim, Chang Sub, Stanton, C. J., J. Appl. Phys. 80, 5899-5908 (1996).

4- Giti A. Khodaparast, Matthew Frazier, Rajeev N. Kini, Kanowkan Nontapot, Tetsuya D. Mishima,
Michael Santos, Bruce W. Wessels, Proc. SPIE 7608, 76080O-1 (2010)

5- M. Bhowmick, T. R. Merritt, K. Nontapot, B. W. Wessels, O. Drachenko, G. A. Khodaparast, Physics
Procedia, (3) 1167-1170 (2010).

6- R. N. Kini , K. Nontapot, G. A. Khodaparast, L. J. Guido, R.E. Welser, J. of Applied Physics 103,
064318 (2008). Selected for April 2008 issue of Virtual Journal of Ultrafast Science.

7- G. A. Khodaparast, J. Kono, Y.H. Matsuda, S. Ikeda, N. Miura, Y. J. Wang, T. Slupinskie, A. Oiwa,
H. Munekata, Y. Sun , F. V. Kyrychenko ,G.D. Sanders , C.J. Stanton, Physcia E, 21 (2004) 978.

8- Matsuda Y. H., Khodaparast G.A., Zudov M.A., Kono J., Y. Sun, F. V. Kyrychenko, G. D. Sanders,
and C. J. Stanton, N. Miura, S. Ikeda, Y. Hashimoto, and S. Katsumoto, H. Munekata, Phys. Rev. B 70
195211-7 (2004).

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